Battery technology dates back at least as early as 1800 and the discovery of the galvanic cell. Since then, numerous investigators have conducted extensive research and experiments in the field of electrochemistry, or battery chemistry, for developing suitable electrodes for different types of batteries. Indeed, most modern-day battery electrodes constitute considerable advance over the electrodes developed by G. Plante in 1859 and batteries employing the modern-day electrodes exhibit decided advantages and improved performance characteristics as compared to the early batteries.
While secondary lead-acid storage batteries have been considerably improved over the years, they still suffer from several significant limitations, especially when the battery is subjected to severe charge-discharge cycling as is often the case in practical use. By severe charge-discharge cycling is meant that the battery is subjected to repeated deep discharges approaching 100 percent of its rated capacity, at high discharge rates, i.e., at discharge rates exceeding the four hour rate (C/4). In order to efficiently recharge conventional lead-acid storage batteries, with little or no deterioration thereto, the charging current must be low as compared to the rated capacity of the battery. Charging at such low currents, however, requires as long as about 8 hours or more to fully charge conventional batteries. Even under the most carefully controlled charge-discharge conditions, the useful life of the present day lead-acid secondary storage battery is limited to several hundred cycles at best, at moderate rates.
The aforementioned and other limitations inherent in the conventional lead-acid type battery is basically due to the types of electrodes which are used in such batteries, primarily the positive electrode, and to a lesser degree the negative electrode. Conventional state-of-the-art lead acid battery electrodes, both positive and negative, are usually made by depositing a layer of electrochemically active material such as, for example, lead oxide on a carrier plate or a grid structure. Because of the highly corrosive nature of the electrolyte (sulfuric acid) in the lead-acid storage battery, the electrode, especially the positive electrode, slowly corrodes and its surface is oxidized thus forming an electron barrier between the carrier plate and the active material thereon. The formation of such barrier limits the current at which the battery can be charged or discharged. As the grid corrodes (is oxidized) further with increased use and life of the battery, the charge and discharge current rates become even more limited and, therefore, the overall efficiency of the battery drops significantly. Eventually, if the battery is used under such conditions, the corrosion of the positive plate and the build up of the barrier become so significant that the battery can no longer be fully charged or discharged under any conditions.
Corrosion of the carrier plate also impairs the mechanical integrity of the electrode structure and can ultimately cause cracking or breaking of the electrode. In addition, even at the early stages of corrosion of the carrier plate, the active material is "shed", especially from the positive electrode, a phenomenon commonly referred to as sulfation. This shedding or sulfation of the electrochemically active material from the electrode is irreversible and the battery will therefore continuously lose its capacity irreversibly as a function of its life.
Since the lead-acid storage battery is often subjected to rather rugged conditions, the carrier plates used in these batteries must be sufficiently strong and heavy to exhibit the necessary mechanical strength. Accordingly, the carrier plates significantly add to the weight of the battery without contributing to its capacity (ampere-hours). In addition, this increased weight generally reduces the realizable energy density (watt-hour per pound or watt-hour per cubic inch) of the battery.
During more than a century since the development of the Plante plates, many investigators have conducted numerous experiments resulting in a plethora of patents and publications directed to improvements in electrodes, or methods of their manufacture, for use in lead-acid storage batteries. Thus, as early as Nov. 19, 1889, Clement Payen was granted a patent for a method of producing a porous crystallized metallic plate for use in lead-acid storage batteries. The Payen method involved subjecting certain metallic salts and a metal to fusion, pouring the fused mass into a mold, chemically reducing the crystallized structure to a metallic state followed by electrolytic action to remove the impurities therefrom.
Another early patent, granted to James Hart Robertson on Sept. 24, 1895, describes a method for making a porous plate or electrode. According to this patent, a metal such as lead is heated to molten condition, to which is then added a granulated or powdered artificial, or natural, porous substance such as pumice-stone, brick-dust, kaolin, coral and the like. The resulting pasty mixture is then kneeded in order to properly incorporate and uniformly disseminate the granular substance throughout the mass, and the mass is then molded, or pressed into a mold of the desired size. The mold is then heated to expand the air cells in the added porous substance, thereby producing air spaces in the molded mass, i.e., an "aerated" molded mass. While the mixture is still in the mold, the mold and the mass therein are subjected to slightly elevated temperatures to obtain a smoother plate and the resulting porous plate is removed from the mold. If the plate is to be used as an electrode in a battery, the Robertson patent discloses that the finished plate may be subjected to such action as may be necessary, e.g., electrolytic action, to produce "active material" through its pore.
The aforementioned patents of Payen and Robertson are but two of numerous patents which relate to improvements in electrode manufacture for use in secondary storage batteries. A more representative list, though by no means exhaustive, includes U.S. Pat. Nos. 415,330; 415,331; 415,348; 415,349; 415,683; 434,458; 440,267; 440,268; 440,269; 440,270; 440,272; 440,273; 440,274; 440, 275; 440,276; 440,277; 538,628; 760,561; 1,749,819; 2,640,864; 2,969,414; 3,113,048; 3,496,020; 3,558,359; 3,582,403; and 3,796,607.
Today, after over 100 years of research and investigations in electrode technology, secondary storage lead-acid batteries still use electrodes made by conventional techniques, and these batteries still suffer from several inherent disadvantages as hereinbefore described. In order to improve the performance characteristics of the lead-acid type batteries, Rudolf R. Hradcovsky and Otto R. Kozak, in their U.S. Pat. No. 4,143,261, issued on Mar. 6, 1979, disclose a positive electrode made of a carrier plate coated with a mixture of polycrystalline and crystalline lead dioxide as the electrolytic active material. Storage batteries incorporating such electrodes exhibit lower internal resistance, improved charge-discharge rates, lower sulphation, higher storage capacity and the ability to draw larger amounts of electric current in a considerably shorter time as compared to conventional state-of-the-art lead-acid batteries.
The present invention contemplates providing a battery having all or most of the improved performance characteristics exhibited by the battery described in the above-mentioned Hradcovsky-Kozak patent without, however, the use of a supporting grid or a carrier plate in making the electrode.
Accordingly, it is an object of the present invention to provide improved electrodes for use in storage cells and batteries.
It is a further object of the present invention to make self-supporting porous electrodes, both negative and positive, for use in secondary storage cells and batteries, particularly those of the lead-acid type.
It is also an object of this invention to provide batteries, particularly storage batteries of the lead-acid type, which have improved performance characteristics by virtue of the incorporation therein of the self-supporting porous electrodes made according to the present invention.
It is another object of this invention to provide a method for making self-supporting porous electrodes, positive and negative, for incorporation into lead-acid type storage cells and batteries.
The foregoing and other objects and features of this invention will be more fully appreciated from the following detailed description of the invention and the accompanying drawings.